Could do with Taff popping in again he has helped me a few times before. Not got a clue myself grey screen on both modes Maybe A bad graphics update from Microsoft. I was going to say try another monitor but as you are getting the grey screen it must be working. Most HP's have a recovery button.F10 or F11 but I would not try that unless someone tells you. maybe it has reset the BIOS to start from another drive but only guessing. Best waiting for taff or someone else to give you the help that you need.
OK, I was rushing! Got me glasses and put the pop away! The screen goes grey which would suggest when you hit the power on button something happens. Can you hear fans working or the hard drive spinning up to speed? If so it would suggest a graphics problem, LCD backlight/inverter failure or a BIOS failure. (Do you see the BIOS Post screen before it attempts to get into windows?
Can you try an external monitor on the laptop to see if that shows anything? Also give me the HP model as well.
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